In the Dynamic Range section of our DSLR reviews we usually look at something that we call ‘RAW headroom’. The RAW headroom could probably be described as the highlight and shadow detail that has been captured in the RAW data but gets lost when a camera’s internal software applies a tone-curve to the RAW image data.

As a rule cameras apply a typical 'S' shaped tone curve to JPEGs to give a visually appealing contrast without harsh clipping of the brightest or darkest tones (the 'shoulders' of the S curve provide a more gentle roll-off than a straight line would). The tone curve applied is often quite steep, sacrificing the tones at the the extremes of highlight and shadow in the pursuit of 'punchy', consumer-friendly out-of-camera results. At the shadow end this is rarely a problem (the very darkest tones are mapped to black, which isn't visually unpleasant and helps reduce noise), but at the highlight end it can mean that what should be a very pale blue sky turns white, or that highlights have harsh edges instead of a gentle gradient. There's no way to get back detail that the camera's tone curve left out of a JPEG, but it is possible to tweak more out of a raw file. Having access to the sensor's full dynamic range also means that you can fix mild under or over exposure by reaching into the headroom.

And so it's very useful to understand what raw headroom is and how you can make use of it to get the maximum out of your images. We regularly receive questions regarding this particular section of our reviews via our feedback system and in the forums, so I thought it would be a good idea to provide some detail about the dpreview approach to raw headroom in a blog article. This isn't a deeply technical post; it's more of a primer on what we do, and why you need to know about the dynamic range hidden away in your raw files.

JPEG vs ‘ACR Best’

For our Dynamic Range test we take an image of a calibrated Stouffer Step Wedge (13 stops total range) which is backlit using a daylight balanced lamp (98 CRI). The exposure is carefully fixed so that wedge no 18 produces the same middle gray. We then feed this image into our in-house software which gives us a Dynamic Range value in stops (EV).

To find out how many extra stops the reviewed camera model is hiding in its raw headroom we measure the dynamic range of a out-of-camera JPEG at default settings and compare the result to the value we get for our ‘ACR Best’ image. The 'ACR Best' image is the test image that gives us the maximum dynamic range result on a particular camera. To create it we tweak the RAW file of our test image (the one of the wedge; whenever possible  we shoot RAW+JPEG) in Adobe Camera RAW until we get an output image that measures maximum Dynamic Range. To achieve this we usually reduce exposure between half a stop and one stop, reduce contrast to a minimum, change the tone curve to ‘linear’ and play a little with the Blacks and Brightness sliders. The optimal settings will be slightly different for each camera though.

When we feed our dynamic range studio test images into our clever software we receive, along with the data, these wedges as an output. The one above illustrates the dynamic range of a Pentax K200D out-of-cam JPEG, the bottom one of the ‘ACR Best’ image.

Out-of-camera JPG

To illustrate the difference that making use of the raw headroom can make to your images I have picked a sample image from the Pentax K200D review and processed the RAW file with three different sets of parameters in Adobe Camera RAW – ACR 4.6 default settings, our ‘ACR Best’ settings and a set of custom parameters.

Below you can see the original out-of-camera JPG which is very vibrant and saturated with good contrast. On the downside it also  shows some fairly large areas of blown highlights on the statue and some minor clipped shadows in the trees to the left (we’ve opened the JPEG in Adobe Camera RAW to make these areas more visible). On this specific image this is as much a metering issue as a dynamic range one. A third of a stop negative exposure compensation when the picture was taken would certainly not have done any harm but on the other hand this makes the image an ideal demonstration object for this blog article. In our K200D review from September this year the camera’s default JPEG gave us a total Dynamic Range of 9.0 stops (3.0 stops highlight range, 6.0 shadow range).

Out-of-camera JPEG in ACR 4.6	Click thumbnail for full-size image
100% crop	100% crop

Adobe Camera RAW with default settings

For comparison purposes I am also showing the image that is produced out of a RAW file by Adobe Camera RAW using the software’s default settings. The colors are slightly less saturated but the contrast rendition is very similar to the out-of-camera JPEG, showing almost identical amounts of clipped highlights and shadows.

ACR 4.6 default settings	Click thumbnail for full-size image
100% crop	100% crop

RAW conversion using ‘ACR Best’ parameters

Let’s have a closer look at the Pentax K200D’s RAW headroom now. Above I described how we determine the RAW headroom by comparing dynamic range of an out-of-camera JPEG to our ‘ACR Best’ image. In the K200D’s case the parameters used to generate the ‘ACR Best’ image are the following:

• Exposure: –0.85
• Blacks: 1
• Contrast: –50
• Curve: Linear
• All other parameters ACR default.
  

Using these parameters on our dynamic range test image we managed to generate an ‘ACR Best’ image that gives us one stop additional dynamic range over the out-of-cam JPEG (additional 0.8 in the highlights, 0.2 in the shadow range). This takes the total achievable dynamic range of the Pentax K200D to 10.0 stops.

‘ACR Best’ parameters	Click thumbnail for full-size image
100% crop	100% crop

What happens if we apply these ‘optimal’ parameters to our real-life sample image? As you can see above the label ‘ACR Best’ is slightly misleading in so far that while the image is optimized in terms of dynamic range – there is maximum detail in both shadows and highlights – it is also very ‘flat’ and dull due to a lack of contrast. It’s obvious that simply applying the parameters that guarantee the optimal dynamic range won't make an image that’s visually pleasing.

Custom parameters for optimized output

What we really want in an image is the best of both worlds, the highlight detail of the ‘ACR Best’ image but also good contrast in the mid-tones and shadows. If you’ve been shooting in RAW that’s not a problem. To achieve the image results you can see below we reduced exposure in RAW conversion by approximately half a stop. This first step brings most of the lost highlight detail back. Then we modified the tone curve to increase contrast in the mid- and darker tones. At the last step we ‘fine-tuned’ the result with a levels correction in Photoshop to make sure we use the entire range of available tones.

ACR 4.6 Custom parameters	Click thumbnail for full-size image
100% crop	100% crop

The table below shows all four images next to each other. You can see that the Custom image shows similar highlight detail to the ‘ACR Best’ image. However, the overall contrast of the picture is much more visually pleasing. Of course, much of this is down to personal taste. If you wanted the Custom image to be more similar to the out-of-camera JPEG for instance, you could simply increase saturation in RAW conversion. However, what these examples are trying to demonstrate is that by shooting in RAW you can preserve highlight detail  and with some minor image modifications create an image that has similar contrast rendition in the mid-tones and shadows to an out-of-camera image. 

Out-of-cam JPEG	ACR 4.6 default settings
‘ACR Best’ parameters	ACR 4.6 Custom parameters